1. Field of the Invention
The present invention relates to realizing protection when the output terminal of a class D audio amplifier short circuits and to preventing failures in the event of clock stoppage with respect to a pulse modulation type electric power amplifier.
2. Description of the Related Art
Class D audio amplifiers that use pulse width modulation (PWM) or delta-sigma modulation have been utilized in recent years. Pulse modulation power amplifiers such as class D amplifiers are energy efficient in comparison to class A or B amplifiers that amplify analog signals, and have been implemented actively in practical applications.
FIG. 14 shows a block diagram of a common pulse modulation type electric power amplifier. This pulse modulation type electric power amplifier basically is constituted of a pulse modulator 1 that receives as input a clock A and an analog input signal B, and an output circuit 3 that performs switching according to a pulse train C output from the pulse modulator 1. An output control circuit 2 composed of a drive circuit, an arithmetic circuit and the like is inserted between the pulse modulator 1 and the output circuit 3 if necessary. The analog input signal B is converted to a pulse train by the pulse modulator 1, and the output circuit 3 performs switching according to the high and low levels of the pulse train. The analog signal recovered by passing the signal from the output circuit 3 through a low pass filter 4 drives a load (speaker) 5.
With class D audio amplifiers, output protection circuitry that prevents current from continuing to flow to an output terminal in the event of an output short circuit has been proposed in order to prevent the failure of speakers or headphones connected to the output terminal or the failure of the class D audio amplifier itself when the output terminal short circuits.
Exemplary protection circuitry disclosed in JP H5-160649A will be described with reference to FIG. 15. With this circuitry, a low pass filter 4 connected to the output terminal is constituted of an inductor and a condenser, and the (mutual) inductor 6 for detecting current is disposed so that a mutual induction effect occurs with the inductor. The current detection inductor 6 is connected to a rectifier 7, and the output of the rectifier 7 is fed to a flip-flop 8 via a low pass filter 4a. 
Current flowing to the output terminal is detected by the current detection inductor 6. This current is converted to a voltage by the rectifier 7 and the low pass filter 4a, and input to the flip-flop 8. The output terminal is regarded as having short circuited when this voltage is greater than or equal to a preset voltage. When this happens, an output stop signal is sent from the flip-flop 8, whereby it is possible to protect the load and the output circuit from failure.
JP 2005-203968A discloses a digital amplifier having the protection device shown in FIG. 16. With this circuitry, the voltage at both ends of an inductor constituting a low pass filter 4 connected to the output terminal is input to an amplifier 10. The output of the amplifier 10 is input to a filter circuit 11 that compensates for deviation in the linearity of the impedance and frequency of the inductor. The output of the filter circuit 11 is input to a microcomputer 13 via a detection circuit 12. An output control circuit 2 is configured so that a pulse train C is input to a drive circuit 2b via a gate circuit 2a, with the output of the microcomputer 13 being fed as the other input of the gate circuit 2a. The output of the low pass filter 4 is fed to a load (speaker) 5 via an output relay 9, which is controlled by the microcomputer 13.
The output of the filter circuit 11 forms a detection signal for current flowing through the inductor of the low pass filter 4, and the microcomputer 13 compares the input current detection signal with a preset reference value. When excess current is flowing through the inductor, the current detection signal exceeds the reference value of the microcomputer 13, and the microcomputer 13 determines that an output short circuit has occurred. When this happens, an output short-circuit detection signal is fed by the microcomputer 13, whereby it is possible to protect the load and the output circuit from failure by the gate circuit of the output control circuit 2 and the output relay 9 operating to stop output.
Also, with pulse modulation type electric power amplifiers, electromagnetic interference (EMI) or physical impacts may cause problems with the clock generating means or with the connection from the clock generating means to the pulse modulator, causing stoppage of the clock signal input to the pulse modulator. As a result, the circuit is fixed in a state in which direct current flows from the output terminal to a load such as speakers or headphones. This direct current can cause the load to heat up and thereby physically harm the user as a result of surpassing the allowable current of the load, or cause speakers or headphones connected to the output terminal or the pulse modulation type electric power amplifier itself to fail. To avoid such situations, it is preferable to provide a means for preventing current from continuing to flow to the output terminal.
However, with conventional protection circuitry such as the above, short circuits are detected by detecting the current flowing to the output circuit, which requires various analog circuitry for measuring the voltage or current, such as an analog amplifier, a high-precision comparator for comparison with a reference voltage or current, and an analog filter for averaging the measured voltage or current. Consequently, a drawback is the complexity and high power consumption of the circuitry. Another problem is the time taken from when a short circuit is determined until output control is implemented because of the control being performed after feeding back the signal of the output terminal.
Further, with a configuration such as that described in JP H5-160649A, difficulties are encountered in embedding the circuit into a semiconductor apparatus due to the use of mutual inductance, with problems of increased mounting area and power loss caused by the mutual inductance.